The invention is based on an injection unit for a plastic injection molding machine.
Such an injection unit has been disclosed by DE 43 44 335 C2. There, two equally large electric motors which are formed as hollow-shaft motors are arranged one behind the other in alignment with the axis of the screw. The screw is firmly connected to a motion spindle which is guided in a spindle nut. The screw joint between the motion spindle and the spindle nut contains balls as rolling bodies. The spindle nut forms the hollow shaft of the one electric motor, which, as viewed from the screw, is located in front of the other electric motor. The hollow shaft of this other electric motor, with a splined journal which lies in the axis of the motion spindle, engages in a spline recess of the motion spindle, so that splined journal and motion spindle are connected to one another in a rotationally locked manner, but the motion spindle can be displaced axially relative to the splined journal.
Essentially two different motion states of the screw can be distinguished from one another during operation. During the plasticization, the rear electric motor drives the screw at a certain rotational speed via the motion spindle and the splined journal. The front electric motor rotates the spindle nut at a rotational speed which differs by a small amount from the rotational speed of the rear electric motor. For example, the front electric motor can rotate slightly slower than the rear electric motor. The difference in rotational speed determines the speed at which the screw travels back. In the process, the difference in rotational speed is controlled in such a way that a certain dynamic pressure is built up and maintained in the screw antechamber into which the plasticized plastic material is delivered. To inject the plastic material, the screw has to be moved forward. To this end, the front electric motor continues to rotate the spindle nut, while the rear electric motor is energized in such a way that it prevents the motion spindle from rotating via the splined journal.
In the known injection unit, two equally large special electric motors which are relatively expensive are used for the drive of the screw.